The present invention relates to a thin film transistor including a so-called dual gate structure for driving the thin film transistor by using a plurality of gate electrodes and a method of manufacturing the thin film transistor. The present invention further relates to a display device including the thin film transistor, and a method of driving the display device.
An organic LED (light emitting diode) has a very high response speed and is a self-emitting device, and thus, it is expected that the application of the organic LED to a display device will allow providing an excellent flat display device having a wide viewing angle. The application of the organic LED to the flat display device replaces a liquid crystal display device. The above-mentioned organic LED is a current-driven element and, thus, the achievement of high-resolution display requires a continuous feed of a current through the organic LED element even during non-selection of a scanning line.
FIG. 9 is a diagram showing a circuit configuration for driving an organic LED, which has been heretofore proposed. The conventional circuit configuration shown in FIG. 9 includes a switching thin film transistor, hereinafter referred to as TFT 80, for generally performing switching, and a driver TFT 84 for driving an organic LED element 82. Switching TFT 80 is driven to be turned on and off in accordance with a signal supplied through a scan line 86 and, thus, supplies to driver TFT 84 a signal supplied through a signal line 88.
Switching TFT 80 is connected to a gate electrode of driver TFT 84 so as to control conductance of driver TFT 84. Under this control, driver TFT 84 supplies a current supplied from a supply line 90 to organic LED element 82 connected to a drain electrode of driver TFT 84 (to drive organic LED element 82). Thus, driver TFT 84 drives organic LED element 82. The conventional circuit shown in FIG. 9 for driving organic LED element 82 also has a configuration in which a charge storage capacitor 92 is connected to a line 94 so as to ensure a stable supply of a current required for organic LED element 82.
Driver TFT 84 supplies a current to organic LED element 82 in accordance with current levels determined by switching TFT 80. In the conventional circuit, the current levels are used under control by which the current levels are changed in about four levels. Time division is used simultaneously with the controlled current levels, thus enabling more levels of control. The use of a change in the current levels allows control on brightness of the organic LED, thereby enabling gray level control on light emission of the organic LED element and enabling high-resolution display.
Gray level control on organic LED element 82 is, in principle, made possible in the following manner: a voltage applied to a gate in performing gray level control on the organic LED element is controlled by using the circuit configuration shown in FIG. 9. The level of current flowing to organic LED element 82 is controlled by using the gate voltage. This relationship is shown in FIG. 10 as a graph which is obtained by plotting a current Ioled flowing to a different organic LED element versus a gate voltage of switching TFT 80, in driving a typical organic LED element.
As shown in FIG. 10, the organic LED element varies in its current-voltage characteristics for each pixel because of various factors. Thus, the same gray level cannot be applied to the organic LED element stably, even when a certain gate voltage Vgate is applied to the organic LED element formed for one pixel each, as shown in FIG. 10. Consequently, a problem exists. When the current level is changed only by changing the gate voltage by use of the circuit configuration shown in FIG. 9 and then gray level control is performed by direct use of the changed current level, the use of only this gray level control does not permit display of precise gray level throughout the whole screen of the display device.
A method of performing gray level control by changing a luminous area of the organic LED element through only on-off control by utilizing the characteristics that a maximum current value Imax and a minimum current value Imin do not vary pixel by pixel even if the current-voltage characteristics vary as shown in FIG. 10; that is, a so-called area gray level control method has been also proposed in order to solve the above-mentioned problem. FIG. 11 shows a drive circuit which has been heretofore proposed to implement the area gray level control method. In the drive circuit shown in FIG. 11, a plurality of organic LED elements 82 are arranged for each pixel 96, and the drive circuit is adapted to be capable of controlling the gray level by controlling the number of organic LED elements 82 to emit light in the luminous area.
FIG. 12 is a plan view showing a configuration in which a plurality of organic LED elements 82 is arranged for each pixel for performing the conventional area gray level control. Organic LED elements 82 are configured in such a manner that a signal is supplied to these organic LED elements from a wiring 98.
Although the method of controlling the gray level by means of the area gray level shown in FIGS. 11 and 12 allows precise gray level control, the method has the problem that the brightness of a display part is decreased because of a decrease in filling rate of organic LED elements 82 per pixel. A driving current may be increased in order to increase the brightness of the display part; however, another problem arises, such that the increase in the driving current causes a reduction in a luminous life of the organic LED element or in a life of the driver TFT. Furthermore, since the need arises to perform on-off control on each organic LED element, it also becomes necessary to increase the number of driving elements such as the driver TFT. Thus, the above-described also leads to a decrease in the filling rate of the organic LED elements per pixel.
For driving the organic LED, a thin film transistor (TFT), as described above, is often used to drive the organic LED for display per pixel. The TFT for use in this case, particularly the driver TFT, is required to be capable of providing a high level of current with stability. Various TFT structures have been heretofore proposed as TFT structures having stabilized current characteristics. A so-called dual gate TFT structure is disclosed in the gazette of Japanese Patent Laid-Open No. Hei 8(1996)-241997, for example. In this structure, gate electrodes are located oppositely to each other so as to sandwich an active layer with insulating layers interposed between the gate electrodes, and one of the gate electrodes is formed shorter than the other gate electrode in a source-drain direction.
The disclosed dual gate TFT exhibits excellent pentode characteristics, and thus stabilizes a drain current even if a power supply voltage varies. However, the dual gate TFT structure disclosed in the gazette of Japanese Patent Laid-Open No. Hei 8(1996)-241997 uses the gate electrode formed shorter than the active layer to form high-resistance portions on both ends of a silicon active layer, thereby achieving excellent pentode characteristics. Therefore, dual gate TFT structure is not sufficient to provide a high current level required for driver TFT of the organic LED.
A TFT including a plurality of gate electrodes, that is, a so-called dual gate TFT, and a display device including dual gate TFT are also disclosed in the gazette of Japanese Patent Laid-Open No. 2000-243963. Dual gate TFT disclosed in the gazette of Japanese Patent Laid-Open No. 2000-243963 comprises a second gate electrode formed to be narrower than a channel length, and thus stabilizes a threshold voltage while preventing a leakage current. However, dual gate TFT disclosed in the gazette of Japanese Patent Laid-Open No. 2000-243963 does not enable gray level control through discrete control on current levels while keeping a high current level required for driver TFT of the organic LED element.
An electro luminescence display device is disclosed in the gazette of Japanese Patent Laid-Open No. 2000-347624. The electro luminescence display device includes a plurality of driving transistors for applying electrical charges stored in a capacitance to an electro luminescence device in accordance with an image signal, and mutual conductance of each driving transistor is set to be expressible as gray levels. Although the electro luminescence device disclosed in the gazette of Japanese Patent Laid-Open No. 2000-347624 can be expressed as gray levels, a plurality of driving transistors must be formed in each pixel, thereby causing a reduction in the filling rate of the organic LED elements. As a result, a problem arises in that the brightness per pixel decreases.
Furthermore, in the gazette of Japanese Patent Laid-Open No. 2000-148087, a TFT structure and a display device are disclosed, the TFT structure including a semiconductor layer, gate insulating films formed on opposite surfaces of the semiconductor layer, and first and second gate electrodes formed on the gate insulating films, respectively. Although the TFT structure disclosed makes it possible to improve yields in manufacturing by increasing a numerical aperture of each pixel, the TFT structure is not adequate to enable gray level control through discrete control on current levels while ensuring a high current level required for the organic LED, and to provide a display device having high brightness and a long life.
The present invention is directed at overcoming the problems as set forth above.